[FFmpeg-devel] [PATCH 2/4] avformat/mxf: get rid of samples per frame array usage

[Baptiste Coudurier]

Hey guys,


> On Mar 3, 2020, at 10:54 AM, Tomas Härdin <tjoppen at acc.umu.se> wrote:
> 
> mån 2020-03-02 klockan 21:35 +0100 skrev Marton Balint:
>> 
>> On Mon, 2 Mar 2020, Tomas Härdin wrote:
>> 
>>> fre 2020-02-28 klockan 01:37 +0100 skrev Marton Balint:
>>>> Signed-off-by: Marton Balint <cus at passwd.hu>
>>>> ---
>>>> libavformat/mxf.c    | 44 ++++----------------------------------------
>>>> libavformat/mxf.h    |  6 ------
>>>> libavformat/mxfdec.c | 23 +++--------------------
>>>> libavformat/mxfenc.c | 24 ++++++------------------
>>>> 4 files changed, 13 insertions(+), 84 deletions(-)
>>>> int ff_mxf_get_content_package_rate(AVRational time_base)
>>>> {
>>>> -    int idx = av_find_nearest_q_idx(time_base, mxf_time_base);
>>>> -    AVRational diff = av_sub_q(time_base, mxf_time_base[idx]);
>>>> -
>>>> -    diff.num = FFABS(diff.num);
>>>> -
>>>> -    if (av_cmp_q(diff, (AVRational){1, 1000}) >= 0)
>>>> -        return -1;
>>>> -
>>>> -    return mxf_content_package_rates[idx];
>>>> +    for (int i = 0; mxf_time_base[i].num; i++)
>>>> +        if (!av_cmp_q(time_base, mxf_time_base[i]))
>>> 
>>> I see this gets rid of the inexact check for an exact one. Approve!
>>> 
>>>> @@ -3307,20 +3307,17 @@ static int mxf_get_next_track_edit_unit(MXFContext *mxf, MXFTrack *track, int64_
>>>> static int64_t mxf_compute_sample_count(MXFContext *mxf, AVStream *st,
>>>>                                         int64_t edit_unit)
>>>> {
>>>> -    int i, total = 0, size = 0;
>>>>     MXFTrack *track = st->priv_data;
>>>>     AVRational time_base = av_inv_q(track->edit_rate);
>>>>     AVRational sample_rate = av_inv_q(st->time_base);
>>>> -    const MXFSamplesPerFrame *spf = NULL;
>>>> -    int64_t sample_count;
>>>> 
>>>>     // For non-audio sample_count equals current edit unit
>>>>     if (st->codecpar->codec_type != AVMEDIA_TYPE_AUDIO)
>>>>         return edit_unit;
>>>> 
>>>> -    if ((sample_rate.num / sample_rate.den) == 48000)
>>>> -        spf = ff_mxf_get_samples_per_frame(mxf->fc, time_base);
>>>> -    if (!spf) {
>>>> +    if ((sample_rate.num / sample_rate.den) == 48000) {
>>>> +        return av_rescale_q(edit_unit, sample_rate, track->edit_rate);
>>> 
>>> Should be OK, per previous rounding argument
>>> 
>>>>                 }
>>>>                 sc->index = INDEX_D10_AUDIO;
>>>>                 sc->container_ul = ((MXFStreamContext*)s->streams[0]->priv_data)->container_ul;
>>>> -                sc->frame_size = 4 + 8 * spf[0].samples_per_frame[0] * 4;
>>>> +                sc->frame_size = 4 + 8 * av_rescale_rnd(st-
>>>>> codecpar->sample_rate, mxf->time_base.num, mxf->time_base.den,
>>>> AV_ROUND_UP) * 4;
>>> 
>>> I was going to say this is only used for CBR video, but closer
>>> inspection reveals it's used to prevent 1/1.001 rate audio packets from
>>> making their way into CBR files. This is a bit surprising to me, since
>>> D-10 supports NTSC (without audio?)
>> 
>> I thought D10 can only be CBR and and it can only use a constant edit unit 
>> size, 1/1.001 audio packet size difference is handled using KLV 
>> padding. So what we compute here is a _maximum_ frame size.
> 
> Now I remember: D-10 is indeed CBR, but at the MXF/CP level. To make it
> CBR you must pad the content packages with fill KLVs as necessary. This
> filling was actually removed by Baptiste a while back, and we had a set
> of patches attempting to fix support for NTSC but sadly the ratecontrol
> in lavc is not up to snuff to support that for files longer than about
> an hour. This also means the video essence needs to be coded at a
> bitrate slightly lower than the D-10 rate, say 49.9 Mbps for 50 Mbps D-
> 10 (+-audio).

IIRC, and correct me if I’m wrong, the VBV delay in D-10 is actually fixed, 
so the essence needs to be CBR at the essence (mpeg-2) level.
Anything else is just broken IMHO.

> I believe at some point there was code for padding the MPEG-2 essence
> with zeroes to make CBR, but that obviously doesn't work with audio due
> to 1601.6 samples per EditUnit

Yeah, the pattern of audio samples is very strict in MXF, and every 5 frames, 
Audio and video durations match perfectly.

— 
Baptiste